1. Field of Invention
The present invention relates to an image sensor, and more generally to a backside illuminated (BSI) image sensor.
2. Description of Related Art
The process of forming a complementary metal oxide semiconductor image sensor (CMOS image sensor, CIS) is compatible with the process of forming a complementary metal oxide semiconductor (CMOS) transistor. Therefore, CMOS image sensors and CMOS transistors can be easily integrated with other peripheral circuits on a single chip to significantly lower the power consumption and the manufacturing cost of image sensors. In recent years, CMOS image sensors have replaced charge-coupled devices (CCD) in the low-end applications and are becoming more and more predominant.
In a typical CMOS image sensor, photosensitive regions are disposed in a substrate, and a back-end-of-line (BEOL), a color filter array and micro-lenses are disposed on the same side of the substrate. Therefore, light passes not only the micro-lenses and the color filter but also the BEOL to reach the photosensitive regions disposed in the substrate. Since metal of the BEOL is non-transparent and blocks the light, the amount of the light reaching the pixel array is very limited.
A backside illuminated (BSI) image sensor is an image sensor for detecting the amount of light from the backside of a substrate. In the BSI image sensor, a BEOL is disposed on the front side of the substrate, and photosensitive regions, a color filter array and micro-lenses are disposed on the backside of the substrate. The light is illuminated on the backside of the substrate and only passes the micro-lenses and the color filter, without passing the BEOL, to reach the photosensitive regions. Light blocking of the BEOL is accordingly avoided, and thus, the sensitivity of each unit region is higher, the quantum efficiency is higher, the interference and the non-uniformity of photo-response are reduced, and the image quality is significantly improved. Due to the development of the pixel structure, CMOS image sensors can compete with CCD sensors, and the applications thereof are not limited to cell phones and are extended to automobiles, medical equipment and monitor cameras. However, a short circuit frequently occurs when the BSI image sensor is tested and packaged with wire bonding technology. electrical short caused by the probing pin, bond wire or bond ball contacting the substrate around the pad opening during the testing and packaging processes. In addition, the pad is too thin to probing contact open and poor bonding issues.